Interactions between different species of bears may have influenced why bears have lived where and how they did historically; why some species went extinct while others still survive--however precariously; and why the ranges of various species do or don't overlap.
There are several cases worth considering.
1) There was minimal sympatry between Arctodus pristinus and A. simus. Pristinus was concentrated in southeastern North America, along with the Florida cave bear, where simus did not occur. Simus occupied higher latitudes, including parts of Alaska and Canada.
2) Brown/grizzly bears did not colonize North America, except the Alaska-Yukon refugium, until A. simus had begun to decline. Yet the American black bear was somewhat sympatric with all 3 species of Tremarctinae. At the southern end of Hudson's Bay, and on the north Atlantic coast, black bear range abutted that of the polar bear within historical times (Lydeckker 1894; Walker 1964). From the late Pleistocene until the 1800's, grizzly bears expanded into black bear range, apparently displacing the smaller species from some habitat, but overall achieving sympatry. Grizzly bears have occupied the Arctic barren grounds from Alaska to Hudson's Bay, including recent range expansions. There is also equivocal evidence that grizzly bears once occupied the barren grounds east of Hudson's Bay, on the Ungava Peninsula (Elton 1954), farther south and east in Ontario (Peterson 1965), and Laborador (Spiess 1976). If eastern barren ground grizzly bears did exist and have been extirpated within the last century, this might explain why black bears are now extending their range into that habitat which they previously avoided (Jonkel & Miller 1970). Alternately, waning numbers of wolves may be critical.
At least where grizzly and black bears are sympatric in YNP and the northern Rockies of Montana, black bears are primarily diurnal, whereas grizzlies are more crepuscular and nocturnal--a trait most extreme for adult males (Mace pers. comm.; Mattson pers. comm.). One possibility is that grizzlies are most active during the darker hours to avoid humans, forcing black bears to be mainly diurnal. That could be tested by comparing activity patterns of black bears in areas where they are vs. aren't sympatric with grizzlies (e.g., western vs. eastern North America). During summers, day length varies greatly with latitude and time of year. So one should check how this influences activity pattern for grizzly vs. black bears.
Emslie & Czaplewski (1985:11) state:
The remains at BC-2 [Labor-of-Love Cave] indicate a co-occurrence of A. simus and U. americanus in this area of Nevada. Sympatric records of these two bears are not unusual and have been documented at several Pleistocene localities including Rancho La Brea and McKittrick (Merrian and Stock, 1925; Schultz, 1938). However, associations of A. simus and U. arctos are rare and have occurred only at Little Box Elder Cave in Wyoming and Fairbanks II in Alaska (Kurten and Anderson, 1974, 1980). Kurten and Anderson (1980) speculate that invading U arctos in the late Wisconsinan may have played a role in the extinction of A. simus through competition. However, this scenario is at odds with Kurten's earlier suggestion that A. simus was a highly predaceous carnivore. If we accept Kurten's interpretations it seems unlikely that the larger and more powerful A. simus could be out-competed by the smaller and less powerful U. arctos. Finally, apparent sympatry of U. americanus and A. simus during the late Pleistocene is more likely if the former species was omnivorous, as it is today, and the latter species was largely herbivorous. In this situation, the invasion of a third species, U. arctos, which can be primarily carnivorous in parts of its range today (Nowak and Paradiso, 1983), may have allowed a sympatric relationship between these three species.
3) At least one European site contains remains of cave, brown, and Asian black bears (Kurte’n 1976:55). However, the 3 species were not typically sympatric and contemporary. During the simultaneous reign of the cave bears and black bears in continental Europe, brown bears were rare. By contrast, brown bears flourished in on British Isles and Asia where cave bears were rare or absent.
4) There is little overlap in the ranges of Asian brown and black bears. Brown bears predominate at higher latitudes, and seem to be displacing Asian black bears from the taiga in the Okhotsk region of the USSR (Stroganov 1969). The lesser overlap between ranges of these species, compared to overlap between American grizzly vs. black bears, may in part reflect the longer period during which the Eurasian species have been adapting to one another. Alternately, adaptation of American black bears to sympatry with Arctodus may have preadapted them to greater coexistence with the grizzly.
5) There is now little overlap between the ranges of sloth and sun bears, although the sloth bear overlaps with Asian black and brown bears, and the sun bear has some overlap with Asian black bears, at least historically.
COEXISTENCE
In Florida, during the Pleistocene, the black bear's omnivorous niche may have minimized food competition with the more herbivorous Tremarctos and the more carnivorous Arctodus. Likewise, the giant panda's preference for bamboo may minimize competition between it and the Asian black bear [and brown bear??]. Preference of polar bears for ringed seal (_______) is likely one factor reducing competition with grizzly/brown and black bears in North America and Asia. But such marked niche separation between bears seems uncommon. Competition may be more typical.
COMPETITION & MUTUAL EXCLUSION
There are anecdotal observations of grizzly bears dominating American black bears during direct confrontations. And grizzly bears apparently limit black bear use of habitats without climbable trees (Herrero 1972). For example, Jonkel (1970?) reported that in northern Montana, black bear range that had its trees burned away was abandoned by black bears and occupied by grizzly bears. ... In Asia too, brown bears seem to be displacing black bears from some habitat.
Indeed, one might expect larger bear species to generally be able to dominate and displace smaller ones. For example, concentration of the lesser short-faced bear and Florida cave bear in the extreme southeastern U.S.A., might have been forced by the giant short-faced bear. Likewise, until Pleistocene megafauna began to decline near the end of the Ice Age, the presence of species such as Arctodus simus in North America might have prevented colonization of this continent by brown bear, wolf (Canis lupus), moose, caribou, various other Siberian mammal generalists, and humans (Geist 1989b).
Geist (1989b) notes that, despite the grizzly's reputation for ferocity, this is based more on defensive than offensive attacks on people. The animal is normally shy and inclined to avoid people, rather than seeking humans as prey. That the dominant carnivore of this continent (bears, felids, and canids) are not be as aggressive towards people as the large felids, canids, and hyenas of Africa and Asia is surprising. Geist suggests that this "shyness" by the grizzly might be a response to encounters with Arctodus and other North American Pleistocene megafauna. So too, the even greater timidity of the European brown bear might have been selected by encounters with the cave bear. (The European brown bear is about the size and temperament of an American black bear.) This hypothesis would be strengthened if evidence were found that Asian brown bears, never subject to domination by a larger ursid species, were much more dangerous to people. Anecdotes suggest that sloth bears are about as aggressive as an American black bear, and that sun bears are as aggressive as a grizzly--sufficiently so to deter even tigers (Domico & Newman 1988). Conclusions await more rigorous data.
The question of modes by which bear species compete with one another brings us back to questions about extinction. Did the presence in continental Europe of both cave bear and Asian black bear leave no open niche for the brown bear, delaying its colonization of that continent until the other species waned? Conversely, did brown bears limit expansion of the cave bear out of Europe and eventually displace cave bears entirely? Geist (1989) favors the former hypothesis, in part because the cave bear dwarfed the brown bear. Archeological evidence, such as cave paintings, indicates that early humans in Europe commonly hunted the brown bear but not the cave bear (Kurte'n 1976)--just as black bear were hunted more commonly than grizzly bear by Native Americans. This suggests that cave bears were not only larger, but more formidable than brown bears. However, relative size and fierceness of the species are not always the decisive factors in competition. Confrontation is not the only mode of competition that can limit overlap between species in habitats suitable for both. There are also at least 4 other factors that should be considered.
Pathogens
One species may be much more vulnerable than the other to certain pathogens. For example, white-tailed deer limit the range of moose Alces alces and caribou Rangifer tarandus, and may even usurp range from these species, because the white-tail is a vector for the brain worm Pneumostrongylus tenuis, which kills moose (Anderson et al. 1964) and caribou. A comparable interaction may have existed between some bears.
Efficiency
More efficient use of food or other habitat features such as den sites.
Food
One might speculate that the ascendency of brown bear over cave bear on the British Isles, and the exceptionally large size of British brown bears, might have been linked to the presence of salmon, which the herbivorous cave bear may not have captured. The smaller size of modern brown bears might likewise reflect their current lack of access to salmon. However, small size also facilitates climbing trees to obtain arboreal foods such as fruit and nuts (e.g., acorns). Oak and beech trees were once abundance in the British Isles. More later about the importance of body size (Ch. 9:__).
Dens
Cave bears denned in caverns, some of which contain remains of hundreds of thousands of individuals (Kurte'n 1976). Except at a few sites in southern England, brown bears remains are seldom found in caves. Both the European and North American U. arctos of today are much more prone to shun caves and dig their own dens than are Asian brown bears, as well as American and Asian black bears. Were caves originally advantageous den sites for all 3 bears in Europe during the Pleistocene, but later a liability? Was Europe invaded by some predator that readily found bears in dens and attacked them there? Were cave dens as warm soil dens?
Greater efficiency in habitat use could promote higher rates of reproduction, survival, and population growth.
c) Population growth rate: Even if a dominant species kills or displaces individual members of another species, the latter may yet predominate if its losses to this cause are more than offset by lower losses to other causes or/and by higher reproduction (we'll never eliminate mosquitos by swatting them).
Infertile Hybrids
Given that a polar bear can interbreed with a grizzly bear, and a spectacled bear with an Asian black bear, interbreeding is certainly plausible for the two sister species of cave and brown bears. Whether their hybrids were fertile is another matter. Kurte'n (19__) hypothesized that CroMagnon people replaced Neanderthals by crossbreeding with them, such that some critical CroMagnon traits were dominant and hybrids exhibiting Neanderthal traits were infertile. The same might apply to competition between bear species. North American (and perhaps South American) Tremarctinae and European cave bear are but a few of the Pleistocene megafauna going extinct near the end of the Ice Age. The causes of this mass extinction aren't known. Various authors (e.g., Martin 1967, 1973, 1984; Martin & Klein 1984) hypothesized that hunting by humans not only accelerated the demise of megafauna, but caused the demise. However, those authors reckoned without the size and probably aggressiveness of Arctodus and other the megafauna -- which, at their prime -- were probably more than a match for humans (Geist 1989), just as grizzly and polar bears have tended to be more than a match for people until the last two centuries.
Social Behavior
Eaton (19__) evaluated relative dominance among large mammalian carnivores in Africa; dominance depended at least as much upon group size as body size. In general, this may have no relevance to bears. Differences in sociality between extant bear species are so slight that they are unlikely to have given any species an advantage over another. However, sociality may have been greater for some extinct species. Evidence that some European caves hosted several hibernating cave bears simultaneously raises the question of whether cave bears formed groups during rest of the year. If so, this could have enhanced their competitive edge over black and brown bears during direct confrontations.
Part IV: INTER-URSID RELATIONS & EXTINCTION
Chapter 16.
INTER-URSID RELATIONS & EXTINCTION